The present invention relates generally to the field of computer data storage systems and more specifically to the field of optical disk data storage systems.
Data storage systems are well known in the art and are commonly used in computer systems where data that is generated or manipulated must be stored and/or retrieved at will. Two major categories of data storage systems that are commonly used to store and retrieve large blocks of data include magnetic disk storage systems and optical disk storage systems. Each category has advantages and disadvantages.
Magnetic disk storage systems are usually used in applications requiring frequent erasure of stored data and replacement with updated data. An advantage of many such systems is that the magnetic storage media is removable, e.g. in floppy disk systems. Only lower capacity and slower access time versions of such systems are generally of this removable type, however. A key disadvantage of magnetic disks is that they are susceptible to mechanical damage as well as inadvertent erasures.
Optical disk systems typically are used where very large amounts of data are to be stored, e.g. 100 megabytes or more. Although optical disks generally are removable, most have the disadvantage of being read-only storage media. The optical disks are prerecorded in a conventional manner, such as using an ablative-pit method in which a precisely focused laser beam burns a depression or pit in the sensitive recording layer of the disk surface. In the optical disc reader, as the disk surface is passed under a light beam in the read mode, a photodetector senses the presence or absence of the depressions and emits electrical signals which are transformed into digital data bits.
Write-once optical disk data storage systems are now available, and offer very high data writing and reading capacity in a removable storage medium. However, such systems have the drawback that once data has been written into a sector of an optical disk, this data cannot be updated by rewriting the sector. Many attempts have been made in the art to enable the high storage capacity of write-once optical disk memories to be utilized efficiently in a computer system in a manner analogous to magnetic disk storage systems, wherein data in a given sector can be updated at will. All such attempts have had inherent problems.
A key problem relates to how the directory or index of stored data on the optical disk is maintained. Without such directory information, it is impossible to selectively access and retrieve data on the disk. In one approach, when data is stored on the optical disk its location is maintained in some sort of directory or index stored on a companion magnetic floppy or magnetic hard disk. This approach has the critical disadvantage of the susceptibility to loss of the directory by erasure or mechanical damage to the magnetic disk, which results in complete and irretrievable loss of the ability to selectively retrieve data stored on the optical disk. This is in addition to the need for keeping each magnetic directory disk physically associated with the optical disk whose directory it is storing. The loss of either of these media renders the other useless.
Another approach has been to store the optical disk directory on the optical disk and, when the disk is first inserted into the disk storage system, initializing an associated magnetic memory to correspond to this directory. As new data is written onto the disc, only the magnetic memory is updated. The entire magnetic memory version of the directory is rewritten onto the optical disk immediately prior to the removal of the disk or the powering down of the system. The disadvantage of this approach is that it uses much disk space since the entire directory may be written on the disk many times, and it is vulnerable to loss of the directory by power interruption before the directory has been rewritten onto the optical disk.
A third alternative method relies on address pointer fields associated with each data segment written to the disk. When data is written onto a particular segment of the optical disk, the associated pointer field remains blank. When an update of the data is desired, it is written to a different segment and the physical address, i.e. the actual physical location on the disk where the updated data has been written, is written to the pointer field of the original data segment. When data is to be read, the pointer field of the original data segment is read to determine the address of the updated information, if any. Because there may be a series of updates to updates in such a pointer field system, trying to read the most recent update of a desired data segment may result in a lengthy sequential examination of a trail of updated data segments before the current data segment is found. The inherent slowness of this system has the serious additional disadvantage of becoming worse and worse as more data updates are made on the optical disk.
What is needed in the art is a system whereby a map of where various data segments are written on the optical disk can be easily maintained and updated on the disk and whereby permanently written data on an optical disk can be used to construct an audit trail of all prior written data segments in the event of the loss of some portion of the disk map. Such a system would increase retrieval speed and provide, in effect, a pseudo-erasable optical disk storage system, a system that emulates an erasable magnetic storage medium while providing the high density storage advantages and data permanence of an optical disk.
The design of optical storage systems presents another inherent problem. Because the recording surface of an optical disk may have defects and because data cannot be retrievably written to a defective surface, it is necessary for the optical system to detect such defects and avoid writing to such areas. Typically, this is accomplished by making two passes over an area on the surface of the disk during each writing operation; once to write the data and once to verify the actual writing. This slows down the operation of the system considerably.
Accordingly, it is the general object of the present invention to provide an improved optical disk data storage system which allows a write-once optical disk to appear to a host system user to be a read/write disk.
Another object of the present invention is to provide an improved method for storing and retrieving data on an optical disk.
A further object of the present invention is to provide an improved optical data storage system which enables rapid identification and retrieval of data at the most recently written physical address on an optical disk corresponding to a selected host system logical address.
Yet another object of the present invention is to provide means for recovering lost directory information for any optical disk that has been written to in a manner according to the present invention.
A still further object of the present invention is to provide an improved optical data storage system in which defects in an optical disk recording surface are automatically detected and bypassed when writing to or recording data from the optical disk.
These and other objects of the present invention will become apparent from the following detailed description and the accompanying drawings.